TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 



133 



ion is indicated by guide lines on Fig. 123. The student should compare 

 this with Figs. 113 and 118, and orient each section with reference to the embryo 

 as a whole. Keep in mind the fact that the transverse sections are drawn from 

 the cephalic surface so that the right side of the figure is the left side of the em- 

 bryo. 



Transverse Section through the Eyes and Otocysts (Fig. 124). The brain is 

 sectioned twice, lengthwise through the myelencephalon, transversely through the fore-brain. 

 The brain wall shows differentiation into three layers: (i) an inner ependymal layer densely 



MttencephaJon 



-MestncephaJon 



Gang. 



Telencephalon 



<ry pit 



FIG. 123. Reconstruction of a 10 mm. pig embryo, showing the chief organs of the left side. The 

 numbered lines indicate the levels of transverse sections shown in the corresponding figures (124-138). 

 For the names of the various structures not lettered see Fig. 118. X 8. Gang, and n. access., ganglion 

 and n. accessorius; Gang. sup. n. 9., superior ganglion of glossopharyngeal nerve; Pulm. artery, pul- 

 monary artery. 



cellular; (2) a middle mantle layer of nerve cells and fibers; (3) an outer marginal layer chiefly 

 fibrous. These same three layers are developed in the spinal cord. A thin vascular layer 

 differentiated from the mesenchyma surrounds the brain wall and is the anlage of the pia 

 mater. The myelencephalon shows three neuromeres in this section. The telencephalon is 

 represented by the paired cerebral hemispheres, their cavities, the lateral ventricles, connecting 

 through the interventricular foramina with the third ventricle of the diencephalon. Close to the 

 ventral wall of the diencephalon is a section of the anterior lobe of the hypophysis (Rathke's 

 pocket). Lateral to the diencephalon is the optic cup and lens vesicle of the eye, which are sec- 

 tioned caudal to the optic stalk. The outer layer of the optic cup forms the thin pigment layer; 

 the inner thicker layer is the nervous layer of the retina. The lens is now a closed vesicle dis- 

 tinct from the overlying corneal ectoderm. 



The large vascular spaces are the cavernous sinuses, which drain by way of the w. capitis 



